Fix potential error loop with output plugins

In cases where a plugin hooks the page layout events, and encounters an
error that halts execution, the error API will inadvertantly trigger the
plugin's error a second time while generating the error output. This
can potentially lead to a fatal PHP error instead of an error message.

To fix this, the event API gains a new function to clear all callbacks
hooked to all events. This allows the error API to safely generate
output without plugin hooks being called.